Science Puzzle
How Can a Gecko Walk Upside Down on Glass?
A gecko can run straight up a window and across the underside of a sheet of glass, upside down, without slipping. Its feet aren't wet or sticky, and they don't leave any mark.
How does a gecko cling to glass?
The Answer
Its toes are covered in millions of microscopic hairs that get so extremely close to the glass that the faint attraction between molecules holds them on. Each hair's grip is tiny, but together they're enormously strong.
Look at a gecko's toe and you'll see soft ridges. Under a microscope each ridge is covered in hairs, nearly half a million on each foot, and each hair splits at the tip into hundreds of even finer branches with flat ends, far smaller than a wavelength of light. Those tips are so small and flexible that they can follow every tiny bump in a surface and get within a few billionths of a metre of it.
At that distance, a weak pull acts between any two molecules, called the van der Waals force. Normally it's far too feeble to notice, because surfaces never get close enough over a large area. A gecko's billions of tips do, and the pulls add up to far more than the animal's weight. In 2000 scientists measured the grip of a single gecko hair and confirmed how it works.
To let go, a gecko doesn't pull its foot straight off. It peels its toes up from the tips, like peeling sticky tape, so only a few hairs let go at a time. Engineers are copying the design to make dry, reusable adhesives.
The principle: Van der Waals forces. Tiny attractions between molecules act only at extremely short distances, and a gecko's millions of microscopic hairs get close enough for them to add up to a strong grip.